In the exploration for hydrocarbons and the exploitation of hydrocarbons from subterranean environs, there is an ongoing need to accurately characterize subterranean reservoirs of interest. Knowing the areal extent, hydrocarbon content, and fluid permeability of a hydrocarbon-bearing reservoir is extremely important to reduce the risk of economic loss and conversely to increase the rate of return on hydrocarbon production from the reservoir. Such information regarding the subterranean reservoir is most readily obtained from one or more wells which are drilled through the reservoir. Drilling rates, drill cuttings, changes in drilling mud composition, and core samples from a well provide the requisite information. Logs generated by passing well logging tools through a well are also a good source of information. Logs provide valuable information concerning the rock and fluid properties of the subterranean reservoir, such as porosity, fluid identification, and shale volume. Exemplary logs include resistivity, gamma ray, density, compressional velocity, shear velocity, and neutron logs.
Since the logs only measure rock and fluid properties up to about one foot from the well bore and the vast majority of the reservoir is not penetrated by wells, the logs are unfortunately only capable of characterizing an extremely small fraction of a reservoir. Furthermore, the act of drilling stresses the rock surrounding the well bore, thereby changing the rock properties and introducing error into measurements obtained by well logging and core analysis. A long standing need exists to accurately characterize rock and fluid properties across substantially the entirety of a subterranean reservoir and, in particular, to accurately characterize rock and fluid properties in regions of the reservoir which are not sampled by wells.
Well data has conventionally been extrapolated away from the well bore to characterize the entirety of the reservoir when well data is limited. Conventional extrapolation techniques depict the subterranean reservoir as a plurality of three-dimensional arrays of blocks or cells which are integrated together to form a three-dimensional model of the reservoir. Typically, the X, Y and Z coordinates of each block are determined in both absolute elevation and stratigraphic surfaces and search algorithms are used to determine relative data points in the vicinity of each block. In addition, the rock properties of each block are assigned by means of estimation methods, such as distance based methods using interpolated averaging methods which are based upon nearby data values and geostatistical methods which account for both the distance and spatial continuity of rock properties.
Seismic surveys have also been used to provide seismic information over the portions of the subterranean reservoir which are not sampled by a well. Impact devices, such as vibratory sources, gas guns, air guns, and weight drops, are employed at the earthen surface or in a well bore as a seismic source to generate shear and compressional waves in the subterranean strata. These waves are transmitted through the subterranean strata, reflected at changes in acoustic impedance, and recorded, usually at the earthen surface, by recording devices placed in an array. This recorded data is typically processed using software which is designed to minimize noise and preserve reflection amplitude. The seismic surveys are ultimately evolved into three-dimensional data sets representing a direct measurement of the surfaces of the rock which define the subterranean reservoir. The data sets are increasingly used to evaluate and map subsurface structures for the purpose of exploring or exploiting oil, gas or mineral reserves. However, seismic data has not generally been utilized in three-dimensional geologic models for any purpose other than to define the top and base of the model. The present invention recognizes a need to more effectively integrate seismic data with geologic models for accurate characterization of subterranean reservoirs.
Accordingly, it is an object of the present invention to provide a method for more accurately predicting quantitative values of rock or fluid properties in a subterranean reservoir by the integrated use of seismic data and dimensional geologic models. It is another object of the present invention to provide a method for predicting quantitative values of rock or fluid properties in a subterranean reservoir which have specific utility for hydrocarbon exploration, enabling the practitioner to more accurately define the magnitude and bounds of a hydrocarbon-bearing reservoir. It is still another object of the present invention to provide a method for predicting quantitative values of rock or fluid properties in a subterranean reservoir which have specific utility for management of the hydrocarbon-bearing reservoir, enabling the practitioner to more closely maximize or otherwise optimize hydrocarbon production from the reservoir. These objects and others are achieved in accordance with the invention described hereafter.